Bulk material handling apparatus are used in connection with the storage and movement of bulk materials such as grain, sand, gravel, coal and the like. Bulk material handling apparatus include primary apparatus and secondary or accessory apparatus. Primary apparatus include conveyors, conveyor transfer points, transfer chutes, bins, silos, hoppers, crushers, screens, flop gates, and associated structures and the like. Accessory apparatus include conveyor belt cleaners, air cannons, industrial vibrators, belt alignment switches, belt tracking devices, overload sensors, and plugged chute sensors, and the like that are used in combination with primary bulk material handling apparatus. For example, accessory apparatus such as air cannons and industrial vibrators are used in combination with primary apparatus such as transfer points, transfer chutes, bins, silos and hoppers to facilitate and control the flow of bulk material through the primary apparatus and improve the performance of the primary apparatus. Similarly, secondary apparatus such as conveyor belt cleaners are used in combination with primary apparatus such as conveyors to improve the performance of the primary apparatus.
Primary apparatus comprising conveyors generally include an endless belt for moving bulk materials from one location to a second location. As the bulk material is discharged from the conveyor belt, a portion of the bulk material often remains adhered to the belt. Secondary apparatus such as conveyor belt cleaners generally have one or more scraper blades that are used to scrape the adherent material from the belt and thereby clean the belt. The scraper blades of a conveyor belt cleaner are typically attached to a cross shaft that extends transversely across the width of the conveyor belt. The conveyor belt cleaner may include one or more tensioning devices that bias the scraper blades into engagement with the conveyor belt with a force that provides a scraping pressure between the scraper blade and the belt. The scraping edge of each scraper blade wears during use due to its scraping engagement with the moving conveyor belt. Tensioners move the scraper blades as the scraper blades wear to maintain the scraper blades in biased scraping engagement with the conveyor belt.
In order to obtain adequate performance from the conveyor belt cleaner, the scraper blades are biased into scraping engagement with the conveyor belt with a selected amount of force to generate a desired scraping or cleaning pressure between the scraper blade and the belt, and that the scraper blades be disposed at a selected cleaning angle with respect to the belt depending upon operating conditions. If the scraper blades are biased against the conveyor belt with an excessive amount of force, this may result in excessive wear to the scraper blades, may cause damage to the conveyor belt, and may cause the tip of the scraper blade to develop an excessively high temperature due to the friction generated between the scraper blade and the moving conveyor belt. If the scraper blades are biased against the conveyor belt with too small of a force, the scraper blades may not effectively clean the conveyor belt.
In addition, the scraper blades may vibrate or chatter against the conveyor belt, thereby potentially damaging the conveyor belt cleaner and/or the belt, and decreasing cleaning efficiency. Scraper blade chatter may be caused by unevenness of the conveyor belt, such as sagging of the belt, defects in the belt, or splices in the belt, and by frictional forces generated between the scraper blade and the moving belt. Chatter typically decreases as scraping pressure increases. Absent chatter, cleaning efficiency generally increases as scraping pressure increases up to the limit where the belt cover strength is exceeded. Thus, the cleaning angle of the scraper blades and the force at which the scraper blades engage the conveyor belt effect vibration or chatter of the scraper blades against the conveyor belt cleaner as well as the cleaning efficiency.
Moreover, every primary and secondary apparatus has a design mass and therefore a characteristic vibration frequency. The characteristic frequency is affected by rotating or moving components such as the belt, gear boxes, motors plus changes that occur over time with the apparatus such as quantity of bulk material conveyed or stored, wear and corrosion or by unwanted buildup of bulk solids in the form of fugitive materials such as carry back, spillage and dust. Changes in the characteristic frequency of an apparatus can be an indication of a change in its mechanical condition or its operating efficiency.
The present disclosure relates to a control system for a bulk material handling secondary apparatus such as a belt cleaner that comprises in essence an “open loop” control, rather than a “closed loop” control. The “open loop” control is configured to respond to a large number of belt operating conditions and generate signals controlling the secondary apparatus only when sensed operating parameters leave an acceptable range, which is determined experimentally after operating the bulk material handling system, and is used as an alternative to “closed loop” systems, such as that disclosed in U.S. Pat. Nos. 7,556,140 and 7,669,708, that continuously monitor the status of the bulk material handling secondary apparatus and adjust the operation of the secondary apparatus to respond to the changes in the operating parameters. An “open loop” system such as that described below responds quickly to changes in operating conditions of the system. The control may sense an abnormal operating condition, and store the abnormal operating condition as an abnormal event. The control may take protective action in the subsequent cycles, if a similar abnormal event is detected.
A “closed loop” system such as that described in U.S. Pat. Nos. 7,556,140 and 7,669,708 may be used in sophisticated, remotely operated and monitored bulk material handling systems. Such a bulk handling system may be a typically high tonnage system utilizing belts of 2 meters to 3 meters wide at belt speeds of 5 meters per second to over 10 meters per second. The control for such a system must be able to respond quickly. For instance, in a “closed loop” control, there may be a delay of 100 millisecond after sensors detect a defect in the belt, generate a control signal, measure feedback, and then take protective action. At a belt speed of 5 meters per second, a 100 millisecond response or delay time equates to approximately 0.5 meters of belt passing before protective action can be initiated. Typical defects in such belts start out as small but significant abnormalities often much smaller than 0.5 meters in length. Often the defect increases in size or severity with each revolution of the belt as the defect encounters the initial cause(s) of the defect on each revolution. Thus a “closed loop” control is effective but often too slow to respond to defects in real time. An “open” loop control such as that described herein is capable of similar controller response times but does not try to react in real time. Rather, the “open loop” system described herein stores in its memory a defect event, and may proactively react during the next revolution and/or may compare the severity of the defect on each revolution and be programmed to react according to instructions. Additionally, even though a “closed loop” control may be programmed to operate in a manner similar to an open loop control, the equipment requirements associated with an “open loop” control are less extensive than for a “closed loop” control. For instance, in a typical “closed loop” control associated with a belt cleaner, there may be a controller associated with each scraper arm of the belt cleaner and a separate cable for each arm enabling operation of each arm controller. As an example, a 2 meter wide belt typically requires a belt cleaner with 11 scraper arms. Thus, a “closed loop” control for the belt cleaner would require 11 cables threaded through the main frame of the belt cleaner to control each actuator. Because the cables carry significant current for actuation of the response means (e.g., a MR damper), and must be rated for mining use, the cables tend to be relatively large in diameter and difficult to install in a belt cleaner. In an “open loop” control, the functionality associated with the arm controller may be significantly reduced or simplified, for instance, by eliminating the need to provide control signals for dampers associated with each arm. The simplification and/or reduction of functionality allows a single bus bar cable system to be used in the belt cleaner of any belt width, which is a significant improvement with no loss of functionality for defects normally encountered in bulk material handling applications.